Method for forming an image on a magnetic composite medium and apparatus therefor

ABSTRACT

In accordance with the invention, an image is formed by applying a local magnetic field to selected regions of a magnetic composite medium comprising columns of magnetic particles distributed in a matrix medium. The particles are &#34;hard&#34; or &#34;semi-hard&#34; magnetic materials in order to retain the latent image as residual magnetism, and the image is developed by exposure to magnetic fluid or powders. The image can be erased by exposure to an AC demagnetizing field or a DC sweep magnet. Preferred apparatus for making such images comprises a sheet of such composite material having a pair of major surfaces with columns of magnetic particles oriented between the surfaces. A local magnetic field, such as a magnetic pen, can be used to write a latent magnetic image on one of the major surfaces. The magnetic columns present the latent image for development at either major surface. In preferred apparatus, one major surface is adapted for magnetic image writing and the other major surface is positioned in sealed relationship with a chamber for exposing the image to magnetic development material. In this arrangement the columns provide a high resolution image on the second surface despite the thickness of the medium between the write and development surfaces.

FIELD OF THE INVENTION

This invention relates to a method for forming an image on a magneticcomposite medium and to apparatus particularly suited for such imageformation.

BACKGROUND OF THE INVENTION

In the Jan. 24, 1992 issue of Science (Vol. 255, p. 446), applicants Jinand Tiefel describe a class of composite materials which are opticallytransparent and, at the same time, electrically conductive. Thesecomposite materials comprise sheets of polymer containing columns ofmagnetic conducting spheres.

Such composite materials have a variety of uses due to their anisotropicelectrical conductivity. They conduct through the thickness of thematerial but not laterally. U.S. Pat. No. 4,644,101 issued to Sungho Jinet al. on Feb. 17, 1987 discloses the use of such materials in apressure-responsive position sensor. The operative principle is thatapplied pressure forces the spheres through any intervening polymer intocontact with one another and through the polymer to the surface. U.S.Pat. No. 5,049,249 shows the use of such material as a means forproviding electrical contact between protruding electrical contactregions. The protruding contacts press on the conductive columns toenhance electrical contact.

The present invention is concerned with the magnetic properties of acomposite medium rather than its electrical properties, and it isspecifically concerned with the use of a composite medium as a materialupon which erasable magnetic images can be written and developed.

SUMMARY OF THE INVENTION

In accordance with the invention, an image is formed by applying a localmagnetic field to selected regions of a magnetic composite mediumcomprising columns of magnetic particles distributed in a matrix medium.The particles are "hard" or "semi-hard" magnetic materials in order toretain the latent image as residual magnetism, and the image isdeveloped by exposure to magnetic fluid or powders. The image can beerased by exposure to an AC demagnetizing field or a DC sweep magnet.Preferred apparatus for making such images comprises a sheet of suchcomposite material having a pair of major surfaces with columns ofmagnetic particles oriented between the surfaces. A local magneticfield, such as a magnetic pen, can be used to write a latent magneticimage on one of the major surfaces. The magnetic columns present thelatent image for development at either major surface. In preferredapparatus, one major surface is adapted for magnetic image writing andthe other major surface is positioned in sealed relationship with achamber for exposing the image to magnetic development material. In thisarrangement the columns provide a high resolution image on the secondsurface despite the thickness of the medium between the write anddevelopment surfaces.

BRIEF DESCRIPTION OF THE DRAWINGS

The nature, advantages and various additional features of the inventionwill appear more fully upon consideration of the illustrativeembodiments now to be described in detail in connection with theaccompanying drawings. In the drawings:

FIG. 1 is a schematic cross section illustrating a method and apreferred apparatus for forming an image in accordance with theinvention; and

FIGS. 2-4 are schematic cross sections of preferred magnetic media forimage formation.

It is to be understood that these drawings are for purposes ofillustrating the concepts of the invention and are not to scale.

DETAILED DESCRIPTION

Referring to the drawings, FIG. 1 is a schematic cross sectionillustrating a method and a preferred apparatus for forming an image inaccordance with the invention. In essence, the method of image formationcomprises the steps of providing a magnetic composite medium 10comprising columns 11 of magnetic particles distributed in a nonmagneticmedium 12, forming a latent image 13 by applying a local magnetic field,as from a magnetic pen 14, to a selected portion of the medium. Thelatent image is developed by applying magnetic fluid or powder 15 andallowing the applied material to accumulate on the image.

In the preferred apparatus for forming such an image, the compositemedium 10 is in the form of a layer having two major surfaces 16 and 17.One major surface, e.g. 16, which can be called a write surface, isadapted to permit the writing of a magnetic image without loss ofmagnetic particles. For example, a wear resistant polymer such aspolyurethane is coated on the surface in sufficient thickness that thecolumnar particles are not extracted by the write pen. The second majorsurface 17, which can be called the development surface, can bepositioned is sealed relation with a development chamber 18 containingthe development fluid 15. The presence of magnetic columns 11 extendingsubstantially between the two major surfaces enables a magnetic imagewritten on surface 16 to be developed as a high resolution image onsurface 17 despite the intervening distance between the two surfaces.Alternatively, the latent image can be developed on the same surface onwhich it is written.

The preferred magnetic composite medium 10 is shown in greater detail inFIG. 2. The composite medium 10 is similar to those described in theaforementioned Jin et al article and patents except that the compositemedium is made of higher coercivity H_(c) magnetic materials withpermanent remanent induction. The earlier composites use soft magneticparticles such as nickel, with typical coercive force (H_(c)) of lessthan 10 O_(e). See R. M. Bozorth, Ferromagnetism, D. Van Nostrand Co.,Inc, New York, 1951, p. 275. Such soft magnetic materials do not retainmuch magnet strength, and they exhibit small or negligible remanentinduction after the applied field is removed. See Metals Handbook, 8thed., Vol. 1. American Society for Metals, 1961, p. 779, and B. D.Cullity, Introduction to Magnetic Materials, Addison-Wesley, Menlo Park(A, 1972, p. 491). They are easily demagnetized especially if themagnetized material has an aspect ratio of less than about 100.

The medium for the present application is made so that the particleswill not escape the write surface. The medium comprises columns 11 ofhigh coercivity magnetic particles 20 distributed in a matrix medium 12.Preferably, a protective layer 21 is disposed on the write surface ofthe medium to prevent the particles 20 from breaking through to thesurface where they could be removed by the magnetic writing pen 14. Ifthe matrix material is an adhesive or rigid material such as epoxy orglass, then the protective layer is not needed.

The particles 20 are magnetic particles made of permanent or semi-hardmagnet materials having H_(c) >100 O_(e). For example, they can bemagnetic alloys such as Nd₂ Fe₁₄ B, Alnico, Fe-Cr-Co, or rare-earthcobalt magnets SmCo₅ or Sm₂ Co₁₇. Alternatively, they can benon-conductive or weakly conductive ferrite magnets such as BaO.6Fe₂ O₃or SrO.6Fe₂ O₃. For permanent image storage, materials having H_(c) >200O_(e) and preferably H_(c) >1000 O_(e) are desirable. Advantageously,the particles are coated with a corrosion resistant material such asgold or silver for corrosion resistance and to reduce light absorption.Typical particle diameters are in the range 0.1 to 2000 micrometers witha preferred range of 10-500 micrometers.

The matrix material 12 can be a polymeric material such as an elastomeror adhesive or it can be a glass. For typical magnetic imageapplications the material can be compliant or rigid. It is important forthe fabrication of medium 10 that the matrix be a material that goesthrough a viscous state before curing or setting. Useful materialsinclude silicone elastomers, epoxies, polyurethane resins and glasses.While transparent media are preferred for a number of applications, thematerial can be lightly colored for decoration. Typical thicknesses areon the order 2-5000 micrometers and preferably 10-500 micrometers.

Medium 10 can be fabricated starting with matrix material 12 in aviscous state. Magnetic particles 20 are demagnetized and mixed with theviscous material in a volume fraction of 0.1-20% but preferably 0.5-5%.After mixing, the material is formed into a layer, as by doctor blading,and, while initially in the viscous state, is subjected to a magneticfield of 50-5000 O_(e), and preferably 200-1000 O_(e) during hardeningor cure. The effect of the magnetic field is to cause the magneticparticles to move in the viscous material into a configuration ofcolumns 11 extending substantially through the medium at randomlocations distributed with substantially uniform density in the medium.

The method of cure or hardening depends on the nature of the matrixmaterial. Polymerizing and thermosetting materials can be heated in anoven. Light sensitive resins can be cured by exposure to radiation ofappropriate frequency, and glasses, thermoplastic materials or inorganiccompounds can be solidified by cooling. After hardening a protectivelayer 21, such as polyurethane, can be formed on the write surface ofthe medium to keep the particles 20 from being extracted during thewrite operation.

The advantages of this medium and apparatus for magnetic image formationare manifold. Resolution is enhanced because it is easier to magnetizeparticles in a column and obtain stronger flux from their ends due tothe improved aspect ratio when the particles are in a columnconfiguration. Moreover the columnar configuration extendingsubstantially through medium 10 permits writing on one surface, e.g. thetop surface, and development of a sharply defined image on the othersurface, e.g. the bottom. This establishes magnetic flux lines close tothe display medium while permitting enclosure of the development mediumaway from the user. This feature can be used to prevent leakage ofmagnetic powders and ferrofluids. Moreover, the use of a columnconfiguration--as distinguished from a random distribution of magneticparticles--permits better transparency for medium 10 than would bepresent for the same content of randomly distributed particles.

Writing of an image can be accomplished by using either a permanentmagnet pen or an electromagnet pen. The pen can be hand-held ormachine-controlled, such as the stylus on an X-Y recorder.

Erasure of a written image can be effected in a variety of ways. Oneapproach is to use a permanent magnet or electromagnet to uniformlymagnetize the write surface. Another approach is to use a permanentmagnet or electromagnet to demagnetize the surface. Yet another approachis to use an erase pen of opposite polarity to erase the image locally.

FIG. 3 is a schematic cross section of an alternative form of the medium10 where the magnetic particles 30 are in the form of magnetic rodshaving a length approximately equal to the medium thickness.

FIG. 4 is a schematic cross section of yet another embodiment where themagnetic particles 40 are spheres having diameters approximately equalto the medium thickness. Fabrication of such a medium is described ingreater detail in applicants' U.S. Pat. No. 4,737,112 issued Apr. 12,1988 and entitled "Anisotropically Conductive Composite Medium".

The fabrication and structure of the invention can be understood ingreater detail by consideration of the following specific example. 3.5%by volume of Sm₂ Co₁₇ magnet particles having diameters in the range200-250 micrometers were mixed in General Electric RTV#615 elastomer.The mixture was then sheeted out as a 600 micrometer sheet onto a glasssubstrate and exposed to a vertical magnetic field (across thethickness) of 300 O_(e) while curing the elastomer at 130° C. for 20min. The resulting medium comprised columns of magnetic particlesextending substantially through the 600 micrometer thickness anddistributed with a substantially uniform average distribution spacing.The medium exhibit a transmittance of about 75% in the visible lightrange.

An image of the letter "A" was then written on the medium by a Nd-Fe-Bmagnetic pen having a 1/16" radius tip (field estimated to be 1600O_(e)). The image was developed by placing a sheet of white paper overthe same and sprinkling Fe powder (25-100 micrometer diameters) onto thesheet and gently tapping. The result was a visible image of the written"A".

An eraser pen with opposite polarity field of 600 O_(e) was moved overthe written "A" on the composite medium, and it was erased. In otherexperiments the image was erased by uniform magnetizing effected bysweeping a vertical field of 3400 O_(e) across the surface.Alternatively, a similar image was erased using demagnetization byapplying an opposite polarity field of 1100 O_(e) across an air gap.

It is to be understood that the above-described embodiments areillustrative of only some of the many possible specific embodimentswhich can represent applications of the principles of the invention.Numerous and varied other arrangements can be made by those skilled inthe art without departing from the spirit and scope of the invention.

We claims:
 1. A method for magnetically forming an image comprising thesteps of:providing a sheet of composite material having a pair of majorsurfaces comprising a non-magnetic matrix material and a plurality ofcolumns of magnetic particles extending between said major surfaces;writing a latent magnetic image on a major surface of said sheet; anddeveloping said latent image by exposing said sheet to magnetic fluid orpowder.
 2. The method of claim 1, wherein said latent image is writtenon one major surface of said sheet and said latent image is developed onthe other major surface.
 3. A magnetic composite medium for magneticimage formation comprising:a layer of non-magnetic matrix materialhaving a pair of major surfaces comprising a plurality of columns ofmagnetic particles extending between said major surfaces, said magneticparticles being comprised of high coercivity magnetic materials havingcoercivity H_(c) >200 O_(c).
 4. A magnetic composite medium according toclaim 3, wherein said magnetic particles are comprised of highcoercivity magnetic materials having H_(c) >200 O_(e).
 5. A magneticcomposite medium according to claim 3, further comprising on one of saidmajor surfaces a protective layer for preventing extraction of saidparticles from said surface.
 6. A magnetic composite medium according toclaim 3, wherein said magnetic particles are rod shaped.
 7. A magneticcomposite medium according to claim 3, wherein said magnetic particlesare spherically shaped.
 8. A method for making a magnetic compositemedium for magnetic image formation comprising the steps of:providing ahardenable, non-magnetic material in a viscous state; mixing in saidmaterial demagnetized particles of magnetic material having H_(c) >100O_(e) ; forming said mixture into a sheet; and exposing said sheet whileinitially in a viscous state to a magnetic field, and causing said sheetto harden.
 9. The method of claim 8, including the step of applying toat least one surface of said sheet a protective layer to preventextraction of magnetic particles from said surface.